P 12 Out-of-plane translatory MEMS actuator with extraordinary large stroke for optical path length modulation in miniaturized FTIR spectrometers Sandner, Thilo; Grasshoff, Thomas; Schenk, Harald; Kenda*, Andreas Fraunhofer Institute for Photonic Microsystems (IPMS); *Carinthian Tech Research AG Maria-Reiche-Str. 2, 119 Dresden, Germany; *Europastraße 4/1, 9524 Villach, Austria E-mail: Thilo.Sandner@ipms.fraunhofer.de Abstract A translatory MOEMS actuator with extraordinary large stroke especially developed for fast optical path length modulation in miniaturized FTIR-spectrometers - is presen ted. A precise translational out-of-plane oscillation at 5 Hz with large stroke of up to 1.2 mm is realized by means of a new suspension design of the comparative large mirror plate with 19.6 mm² aperture using four pantographs. The MOEMS device is driven electro - statically resonant and is manufactured in a CMOS compatible SOI process. Up to ± 6 µm amplitude (typically 1mm stroke) has been measured in vacuum of 3 Pa and 5 V driving voltage for an optimized pantograph design enabling reduced gas damping and higher driving efficiency. Keywords: Optical SOI-MEMS, translatory micro mirror, optical path length modulation, Fouriertransform infrared spectrometers, optical vacuum packaging 1 Introduction Fourier Transform Infrared (FT-IR) spectroscopy is a widely used method to analyze different materials - organic and inorganic. Current FT-IR spectrometers are large, usually static, and are operated by qualified personnel. By using translational MOEMS devices for optical path length modulation instead of conventional highly shock sensitive mirror drives a new class of miniaturized, robust, high speed and cost efficient FTIR-systems can be addressed. An early approach of a miniaturized MEMS based FTIR spectrometer has been developed in the past by IPMS and the CTR [1]. It was a combination of classical infrared optics with a translatory 5 khz MEMS mirror using a folded bending spring mechanism. Due to the limited amplitude of ± µm a spectral resolution of 3 cm -1 was realized allowing dynamic FTIR measurements in the ms-range [2]. To enhance the stroke IPMS introduced a first translational MEMS device with two pantograph mirror suspensions originally designed for larger stroke of 5 µm. But due to superimposed parasitic torsional modes only ± 14 µm amplitude could be measured [2]. In this paper, we now present an optimized MEMS device which overcomes the previous limitations enabling an extraordinary large stroke of 1 mm. The novel translatory MOEMS actuator was specially designed to enable a miniaturized MEMS based FTIR spectrometer with improved system performance of 5 cm -1 spectral resolution ( = 2.5 16 µm), SNR > and fast operation of 5 scans / sec. Hence, a large mirror aperture of 5 mm, enhanced amplitude of ± 5 µm and a small dynamic deformation of < /4 is required. Due to the significant viscous gas damping in normal ambient the translatory MEMS devices have to operate in vacuum requiring a long term stable optical vacuum package with broadband IR window. The paper discusses the design, fabrication and experimental characteristics of the novel translatory MEMS actuator including first results of the optical vacuum packaging. To realize a large stroke of the mirror plate a pantograph like suspension was chosen. The new translatory MEMS actuator consists of four symmetric pantograph suspensions in contrast to two pantographs used for a previous MEMS design, where only ± 14 µm amplitude could be achieved due to parasitic tilt modes. One single pantograph consists of six torsional springs two springs arranged on the same axis and connected by stiff levers. The torsional springs are used us deflectable elements instead of bending springs which reduces significantly parasitic mirror deformation due to mechanical stress. Due to the optimized mechanical design using 4 pantograph suspensions the new translatory MEMS actuator can provide a precise out-of-plane translation with ± 5 µm amplitude in vacuum of 3 Pa at 5 V. This enables a completely new family of low cost handheld FTIR analyzers with a spectral resolution of up to 5cm -1, scans/s and SNR > e.g. applied by individuals for ad-hock inspection of food or environmental parameters. The work was performed in the context of the FP7 project MEMFIS. S E N S O R + T E S T C o n f e r e n c e s 2 1 1 I R S P r o c e e d i n g s 1 5 1
2 Translatory Mems Mirror The novel translatory MOEMS actuator was specially designed to enable a miniaturized MEMS based FTIR spectrometer with improved system performance of 5 cm -1 spectral resolution ( =2.5 16µm), SNR > and fast operation of 5 scans / sec. Hence, a large mirror aperture of 5 mm, enhanced amplitude of ± 5 µm and a small dynamic deformation of < /4 is required. 2.1 Large stroke MEMS design The MEMS design have been developed within an iterative process using FEA simulations of single and coupled physical domains and transient simulations of the resulting dynamic behaviors (e.g. frequency response curves) using reduced order models. For the translatory MEMS design the following general specifications and boundary conditions have been considered. Specification and Boundary Conditions of MEMS design: Resonance frequency : f = 5 Hz Mirror diameter: D = 5 mm Mechanical amplitude: z = ± 5 µm Dynamic mirror deformation: δ p-p < λ / 1 ( λ = 25 m δ p-p < 25 nm ) Max. mechanical stress: s1 1.4 GPa Max. MEMS dimensions: l x w = 1 µm x 1 µm Max. acceleration a max = g Reflectance of mirror R @ λ = 2.5 µm 16 µm 95 % c) d) Figure 1. Translatory MEMS design with pantograph suspension of oscillating mirror plate; (a) basic design of the pantograph suspension (detail of a 1/4 plate), (b) previous translatory MEMS device with two pantograph suspensions at 32 µm deflection [2], c) FEA model of new MEMS with four pantograph suspension, d) photograph of new MEMS device @ 4µm pre-deflected. To realize a large stroke of the mirror plate a pantograph like suspension was chosen. Here, torsional springs are used us deflectable elements instead of bending springs which reduces significantly parasitic mirror deformation due to mechanical stress. For the translatory MEMS devices - presented in this paper - the mirror plate is supported symmetrically by four pantograph suspensions (see figure 1) in contrast to two pantographs used for a first pantograph MEMS design [2], where only ± 14 µm amplitude could be achieved due to parasitic tilt modes [2]. One single pantograph consists of six torsional springs two springs arranged on the same axis and connected by stiff leavers. By using four symmetric pantograph suspensions an excellent mode separation for precise translation could be realized for the new translatory MEMS [4]. Beside the modal analysis the following main results were simulated: For a 5mm mirror MEMS device a dynamic mirror deformation of 433 nm (p-p-value) was simulated. To reduce the dynamic mirror deformation bellow the limit of λ/1 = 25nm (p-p) an alternative MEMS design with slightly reduced mirror diameter of 4.2mm was developed, which results in a smaller dynamic deformation of 22 nm at ± 5 µm amplitude. The required driving voltage was simulated for a vacuum pressure of 3 Pa to maximal U D 11 V bellow the electrostatic stability (pull-in) voltage of minimum U pull-in 118 V. The maximal mechanical stresses of typical 1. GPa (in the worst case 1.24 GPa) where simulated at maximal mech. deflection of ± 5 µm using nonlinear FEA simulations, which is bellow the limit of 1.4 GPa. Due to the large moment of inertia about 2 GPa stress occurs at g acceleration, but for the majority of practical applications it is not crucial. S E N S O R + T E S T C o n f e r e n c e s 2 1 1 I R S P r o c e e d i n g s 1 5 2
2.2 Fabrication The translatory MOEMS device are manufactured in a CMOS compatible SOI process [3] using a highly p-doped device layer of 75 µm. The translatory MEMS actuators are driven electro-statically resonant using in-plane vertical comb drives [3] located on each of the 4 pantographs for optimized driving efficiency (see figure 2). To enable areas of different electrical potential within the same structural SOI plane vertical insulations are required. In contrast to the standard scanner technology of IPMS where filled insulation trenches are used typically now for the new translatory MEMS devices reported in this paper only open vertical trenches have been used for electrical isolation of out-of-plane comb drives to simplify the MEMS process. Photographs of finally fabricated translatory MEMS devices are shown in figure 1d and figure 2. For electrical characterization and later vacuum packaging the MEMS chips are chip bonded on a ceramic wiring board (see figure 7, left). Spring 2 vertical trench isolation Spring 3 a) Spring 1 b) Figure 2: Photographs of four pantograph suspension; un-deflected (a) and SEM of 4 µm pre-deflected translatory MEMS (b). 2.3 Experimental results The frequency response behavior of the translatory MEMS devices have been measured by means of a MICHELSON interferometer setup [2]. The devices were measured with a down frequency sweep at varied driving voltages and pressures. To vary the ambient vacuum pressure the MEMS sample under test was encapsulated in a small vacuum camber. In the experimental setup the vacuum pressure could be varied from ambient pressure down to 1 Pa as the minimum. The translatory MEMS devices are driven in open loop operation [2] with a pulsed driving voltage of 5 % duty cycle and a pulse frequency twice the mechanical oscillation. The experimental results of translatory MEMS (1st prototype design) with large mirror diameter of D = 5mm is exemplary shown in figure 3. 4 HA4_1_9_3V_3Pa_sinus2 35 HA3_2_17_sinus_3V_3Pa 35 HA4_1_9_4V_3Pa_sinus HA4_1_9_5V_3Pa_sinus HA4_1_9_6V_3Pa_sinus 25 HA3_2_17_sinus_4V_3Pa HA3_2_17_sinus_5V_3Pa HA3_2_17_sinus_6V_3Pa 25 15 15 5 5 132 133 134 135 136 137 138 139 14 141 142 143 Frequency / Hz 145 147 149 151 153 155 157 159 Frequency / Hz Figure 3: Experimental results of translation MEMS with D = 5mm ( 1 st prototype); pulse frequency response curves measured for varied driving voltages under vacuum conditions of 3 Pa (left); Experimental issues of 1 st prototypes are obvious to be eliminated by redesign: (left) regions of instable oscillation (driven with pulse voltage), (right) stiffening effect. For the translatory MEMS with 5mm mirror diameter a amplitude of maximal ± 37 µm was measured at a vacuum pressure of 3 Pa and a driving voltage of U = 6 V. Hence, for this MEMS device the FTS specification of at least ± 5 µm amplitude was not achieved at minimum pressure of 3 Pa due to higher viscous damping, where no oscillation occurs in normal ambient even for larger driving voltages S E N S O R + T E S T C o n f e r e n c e s 2 1 1 I R S P r o c e e d i n g s 1 5 3
S of up to V. In addition several experimental issues were observed at larger deflection > µm like regions of instable oscillation (see figure 3a) and stiffening effect. The reason is assumed to be related to Eigen modes of the pantograph levers. The regions instable oscillations within the frequency response curves could be significantly reduced by using a pure harmonic driving voltage of twice the oscillation frequency instead of a square wave. Hence a redesign was required for the 5mm translatory MEMS mirror to reduce damping and to eliminate parasitic effects. In contrast to the results of the 5 mm mirror device the alternative translatory MEMS device with slightly reduced mirror diameter of D = 4.2 mm have shown an improved overall performance. The frequency response curves measured at 3 Pa and varied driving voltages are shown in figure 4 (left). Here, continues increase of amplitude and reduced resonance frequency is obvious for increased driving voltages - as expected from the dynamic simulations. Typically, an amplitude of ± 5 µm was achieved at driving voltages of U = 7 8 V at minimal pressure of 3 Pa. Amplitude of ± 7 µm was measured as the maximum. No parasitic oscillations were observed within the specified amplitude range of ± 5 µm. In addition the dynamic tilt error of the oscillating MES device estimated to 1 arcsec using a stroboscopic autocollimator setup. The dynamic mirror deformation which is maximal at the turning points of mirror oscillation was simulated to δ p-p = 22nm (λ/11.4). Hence, beside the slightly reduced mirror diameter of 4.2 mm the full specification required for a miniaturized FTIR spectrometer as successfully achieved for this alternative design of a large stroke translatory MEMS device. Figure 4 (right) shows the pressure dependency of frequency response measured at 7 V for a 4.2 mm translatory mirror device. Here, vacuum pressure was varied between 2. Pa. For the 4.2 mm mirror a reduced damping was measured, but also no oscillation occurred in normal ambient. To achieve the full amplitude of ± 5 µm a vacuum of 5 Pa and 7 9 V driving voltage is required. 5 5 Amplitude / µm 45 4 35 25 3V 35V 4V 45V 5V 55V 6V 65V 8V 45 4 35 25 2Pa Pa 5Pa Pa 15 15 5 5 128 13 132 134 136 138 14 142 Pulse frequency / Hz 128 129 13 131 132 133 Pulse frequency / Hz Figure 4: Experimental results of translation MEMS with reduced mirror size of D = 4.2 mm ( 1 st prototype); pulse frequency response curves measured for varied driving voltages under vacuum conditions of 3 Pa (left); frequency response for varied vacuum pressure measured at 7 V pulse voltage. 2.4 Optimization of large stroke MEMS design Beside the general specifications of MEMFIS the redesigned MEMS devices should avoid the issues of 1 st translatory MEMS demonstrators with 5mm mirror diameter: Avoiding of any parasitic oscillation by improvement of mode separation of pantograph mirror suspension, supposed to be responsible of shown reliability issues, Realization of full translatory amplitude z max = ± 5 µm also for a large mirror diameter of 5mm. Therefore damping of the oscillating translatory MEMS device at constant ambient pressure has to be reduced by reducing size and area of moving parts of the pantograph mirror suspension. In figure 5 the geometry of the optimized translation MEMS device (2 nd prototype with D = 5mm) is shown in comparison to the previous 1 st prototype MEMS device. A significant smaller geometry of the pantograph levers is obvious for the optimized MEMS design (2 nd prototype). The size reduction was realized by increasing the torsional deflection of the spring suspensions enabling the same stroke using a larger transformation factor of torsion to translation. Due to the smaller size and area of the pantograph suspension a significant reduction of viscous gas damping is expected. In addition due to the more compact pantograph geometry a larger driving capacity and improved driving efficiency could be realized. S E N S O R + T E S T C R o n f e r e n c e s 2 1 1 1 I P r o c e e d i n g s 1 5 4
S a) b) a) b) Comb drives Figure 5: Optimized translation MEMS device (a) - 2 nd prototype with D=5mm & stroke of 1mm in comparison to (b) 1 st prototype device, the smaller pantograph lever design (a) enables reduced damping and higher driving efficiency. 2.5 Experimental results of optimized MEMS design In figure 6 the experimental results of the optimized translation MEMS devices (2 nd prototype design) with 5 mm large mirror diameter is shown. The frequency response measured for varied driving voltages under vacuum conditions of 3 Pa is shown in figure 6 (left) for a driving by means of the stationary comb electrodes. A significant improvement of the frequency response behavior is obvious in comparison to the 1 st prototype device with D = 5 mm (see figure 3). Also significant larger amplitudes of typically ± 5 µm (up to ± 6 µm, see figure 6, left) could be measured at same pressure of 3 Pa for reduced driving voltage of U = 5 V in comparison to only ± 37 µm @ U = 6 V enabled for the 1 st prototype device (see figure 3). In addition, no issues with parasitic oscillations or stiffening effects were observed for the optimized MEMS design. Figure 6 (right) shows the influence of vacuum pressure on amplitude response measured at 4 V driving voltage. A significant lower damping and driving voltage is obvious. In contrast to the 1 st prototype design the optimized MEMS design enables also an oscillation in normal ambient. Even for 5mm mirror device ± 8 µm amplitude were measured in normal ambient at 4 V and ± 46 µm at 3 Pa. Hence, the optimized translatory MEMS devices enable a reduced effort for vacuum package due to the significant lower damping & driving voltage. 7 6 1Pa 6 5 4 3V @ 3Pa 4V @ 3Pa 5V @ 3Pa 5 4 3Pa 5Pa Pa 5Pa Pa 5Pa ambient pressure 91 915 92 925 93 935 94 945 95 955 Pulse frequency / Hz 91 92 93 94 95 96 97 Frequency / Hz Figure 6: Experimental results of optimized translation MEMS with D = 5 mm & stroke of 1mm ( 2 nd prototype); frequency response curves measured for varied driving voltages under vacuum conditions of 3 Pa (left) driven by movable comb electrodes; pulse frequency response of translatory MEMS measured at 4V driving voltage and varied vacuum pressure. 3 Outlook The large stroke translatory MEMS devices, presented in this paper, will be used to build an improved version of a miniaturized MEMS based FTIR spectrometer [2]. The system concept is based on a miniaturized conventional optical Michelson setup combined with the new translatory MEMS devices used for optical path length modulation (see figure 7). S E N S O R + T E S T C R o n f e r e n c e s 2 1 1 1 I P r o c e e d i n g s 1 5 5
S Figure 7: Optical vacuum package of translatory MEMS (left); Miniaturized MEMS based FTIR spectrometer, optical layout and block diagram of the signal path. To guarantee the full typically ± 5 µm amplitude a long term the MEMS component has to operate in vacuum of < 5 Pa us discussed before. Hence, the MOEMS device has to enclose into a sealed optical vacuum package so that the external vacuum supply used in the first FTS prototype [1] will become obsolete. Currently, an optical MEMS vacuum package is under development especially designed for a broad IR spectral range (λ = 2.5 µm 16 µm). The optical vacuum package is based on a hybrid chip assembly using a ceramic wiring board and hermetic soldering of ZnSe window and metal can housing (see figure 7). It has been shown in this paper that the requirements for optical vacuum packaging can be significantly reduced by using a optimized MEMS design with reduced gas damping. The experimental results of optical vacuum packaging and final system integration into a miniaturized FTIR spectrometer will be published elsewhere sonly. These actual developments should lead to a sensitive, reliable and easy to use stand alone FTIR spectrometer qualified for industrial applications e.g. process control. 4 Conclusions In this paper we present a resonant driven translatory MEMS mirror enabling extraordinary large stroke of up to 1 mm for optical path length modulation. Due to an optimized mechanical design using four pantograph suspensions of the 5 mm large mirror plate previous problems with mode separation could be solved. Now, the new translatory MEMS actuator can provide a precise out-of-plane translation of ± 5 µm amplitude in vacuum of 5 Pa, typically. A significant lower damping could be realized for an optimized MEMS design were larger torsional spring deflections are used to enable a more compact pantograph geometry. Experimentally up to 1 µm stroke @ 3 Pa & 5 V were achieved even for a 5 mm aperture. Now also an oscillation in normal atmosphere was realized, so fare ± 8 µm were measured in normal pressure at only 4 V. The new translatory MEMS devices are very promising for miniaturized FTS, to replaces expensive, complex and shock sensitive drives. The versatility and ruggedness of a MOEMS based FTS makes it ideal for process control and applications in harsh environments (e.g. surveillance of fast reactions due to the high scan rates). This enables a completely new family of low cost handheld FTIR analyzers with a spectral resolution of up to 5 cm -1, scans/s and SNR > e.g. applied by individuals for ad-hock inspection of food or environmental parameters. We acknowledge financial support by the European Commission in context of the FP7 project MEMFIS. References [1] A. Kenda, et al., Application of a micromachined translatory actuator to an optical FTIR spectrometer, Proc. SPIE 6186, pp. 78-88, 6 [2] T. Sandner, et al., Translatory MEMS actuators for optical path length modulation in miniaturized fourier-transform infrared spectrometers, Journal of micro/nanolithography, MEMS and MOEMS vol. 7 ( 2), pp. 26: 1-12, 8 [3] H. Schenk, et al., Large Deflection Micromechanical Scanning Mirrors for Linear Scans and Pattern Generation, Journal of Selected Topics of Quantum Electronics vol. 6, pp. 715-722, [4] T. Sandner, et al., Out-of-plane translatory MEMS actuator with extraordinary large stroke for optical path length modulation, Proc. SPIE, MOEMS & Miniaturized Systems, San Francisco, 211 S E N S O R + T E S T C R o n f e r e n c e s 2 1 1 1 I P r o c e e d i n g s 1 5 6